1. Coagulation in Industrial water Treatment
J(Hans) van Leeuwen and Samir K Khanal
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water treatment

2. Why coagulation is needed
Various sizes of particles in raw water
Particle diameter Type Settling velocity mm
10 Pebble m/s
1 Course sand m/s
Fine sand 0.6 m/min
Silt m/d
(10 micron) Large colloids 0.3 m/year
(1 nano) Small colloids 3 m/million year
G r a v i t y s e t t l i n g
Colloids – so small, gravity settling not possible
Metal precipitates are usually colloidal
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3. COAGULATION Size of colloids - light waves Brownian motion
Stability of colloids
Removal of colloidal substances from water
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4. - Colloid Stability Colloid Destabilization
H2O
Colloid
Colloids have a net negative surface charge
Electrostatic force prevents them from agglomeration
Brownian motion keeps the colloids in suspension -
Repulsion
Colloid - A
Colloid - B
Impossible to remove colloids by gravity
settling
Colloid Destabilization
Colloids can be destabilized by charge neutralization :
Positively charged ions (Na+, Mg2+, Al3+, Fe3+ etc.) neutralize the
colloidal negative charges and thus destabilize them.
With destabilization, colloids aggregate in size and start to settle
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5. What is Coagulation?
Coagulation is the destabilization of colloids by addition of chemicals that reduce the negative charges
The chemicals are known as coagulants, usually higher valence cationic salt (Al3+, Fe3+ etc.) -
Rapid mixing
is required
to disperse
the coagulant
throughout
the liquid
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6. FORCES ACTING ON COLLOIDS
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7. Aluminum sulfate: Al2(SO4)3.14 H2O
Relative coagulating power of cations
Na+ = 1; Mg2+ = 30
Al3+ > 1000; Fe3+ > 1000
Typical coagulants
Aluminum sulfate: Al2(SO4)3.14 H2O
Polyaluminum Chloride (PAC): Al2(OH)3Cl3
Iron salt- Ferric Sulfate: Fe2(SO4)3
Iron salt- Ferric Chloride: Fe2Cl3
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8. Aluminum Chemistry Al2(SO4)3.14 H2O  2Al(OH)3+ 8H2O + 3H2SO4-2
Al2(SO4)3.14 H2O + 6HCO3-  2Al(OH)3+ 6CO2 + 14H2O + 3SO4-2
With alum addition, what does happen to water pH?
(Optimum pH: 5.5 – 6.5)
1 mole of alum consumes 6 moles of bicarbonate (HCO3-)
If alkalinity is not enough, pH will reduce greatly due to sulfuric acid formation.
Lime or sodium carbonate may be needed to neutralize the acid.
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9. Alkalinity calculation
If 200 mg/L of alum to be added to achieve complete coagulation. How much alkalinity is consumed in mg/L as CaCO3?
Al2(SO4)3.14 H2O + 6HCO3-  2Al(OH)3+ 6CO2 + 14H2O + 3SO4-2
594 mg mg
594 mg alum consumes mg HCO3-
200 mg alum will consume (366/594) x 200 mg HCO3-
= 123 mg HCO3-
Alkalinity in mg/L as CaCO = 123 x (50/61)
= 101 mg/L as CaCO3
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10. Exercise: Alkalinity calculation
Iron Chemistry
FeCl3+ 3HCO3-  Fe(OH)3+ 3CO2 + 3Cl-
With Iron salt addition, what does happen to water pH?
(Wider pH range of: 4 – 9; Best pH range of 4.5 – 5.5)
1 mole of FeCl3 consumes 3 moles of bicarbonate (HCO3-)
If alkalinity is not enough, pH will reduce greatly due to hydrochloric acid formation. Lime or sodium carbonate may be needed to neutralize the acid. Lime is the cheapest one.
Exercise: Alkalinity calculation
If 200 mg/L of ferric chloride to be added to achieve complete coagulation. How much alkalinity is consumed in mg/L as CaCO3?
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11. Jar Tests
The jar test – a laboratory procedure to determine the optimum pH
and the optimum coagulant dose
A jar test simulates the coagulation and flocculation processes
Determination of optimum pH
Fill the jars with raw water sample
(500 or 1000 mL) – usually 6 jars
Adjust pH of the jars while mixing
using H2SO4 or NaOH/lime
(pH: 5.0; 5.5; 6.0; 6.5; 7.0; 7.5)
Add same dose of the selected
coagulant (alum or iron) to each jar
(Coagulant dose: 5 or 10 mg/L)
Jar Test
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12. Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid mix
helps to disperse the coagulant throughout each container
Reduce the stirring speed to 25 to 30 rpm and continue mixing for
15 to 20 mins
This slower mixing speed helps promote floc formation by
enhancing particle collisions which lead to larger flocs
Turn off the mixers and allow
flocs to settle for 30 to 45 mins
Measure the final residual
turbidity in each jar
Plot residual turbidity against pH
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Jar Test set-up

13. The pH with the lowest residual turbidity will be the optimum pH
Residual turbidity Versus pH
Optimum pH: 6.3
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14. Determination of optimum coagulant dose
Fill jars
Adjust pH of all jars at
optimum (6.3 found from first test)
while mixing using H2SO4 or
NaOH/lime
Add different doses of the selected
coagulant (alum or iron) to each jar
(Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L)
Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid
mix helps to disperse the coagulant throughout each container
Reduce the stirring speed to 25 to 30 rpm and continue mixing
for 15 to 20 mins
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15. This slower mixing speed helps promote floc formation by
enhancing particle collisions which lead to larger flocs
Turn off the mixers and allow flocs to settle for 30 to 45 mins
Measure the final residual turbidity in each jar
Plot residual turbidity against coagulant dose
Coagulant Dose mg/L
Optimum coagulant dose: 12.5 mg/L
The coagulant dose with
the lowest residual
turbidity will be the
optimum coagulant dose
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16. Rapid Mixing (Coagulation)
To uniformly mix the coagulant with colloidal matters present
in raw water so as to bring about colloidal destabilization
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17. How to achieve rapid mixing
Horizontal baffled tank
The water flows in a horizontal
direction. The baffle walls help
to create turbulence when the water hits the surface and thus facilitate mixing L W
Plan view (horizontal flow)
Vertical baffled tank
The water flows in a vertical direction. The baffle walls help to create turbulence when the water hits the surface and thus facilitate mixing H L
Isometric View (vertical flow)
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18. Hydraulic Jump: Hydraulic Jump creates turbulence and
thus help better mixing.
Coagulant
In-line flash mixing
Mechanical mixing
Back mix impeller flat-blade impeller
Inflow
Chemical
feeding
Chemical
feeding
Inflow
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25. COAGULANT AIDS Other substances than coagulants used:
- Clay minerals
- Silicates
Polymers
Polymers are often
either anionic or
cationic to aid
coagulation.
Polymers also
reinforce flocs
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26. Typical layout of a water treatment plant
Flocculation and its applications in water treatment
                                                                                                                                                                                                                                                                                                                                             
Typical layout of a water treatment plant
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